Embryology plays a vital role in the clear and thorough understanding of how the human body comes into being and the underlying pathophysiology of congenital disorders that affect neonates and people at different stages of their lives. The study of embryology has proven to play a significant role in the clinical understanding and management of such embryological disorders.

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Once you have the sperm arrive to where the egg is, this is usually in the ampulla region of thefallopian tubes, although it can occur elsewhere but that is the typical locale. The fertilizationprocess is multiple step in nature. To do this, we will take these steps one at a time as we gothrough looking at this overall response. The sperm, I think, you can recognize here in thepicture. The egg is the purple shape with the couple rings around it. The rings are importantwhen I go through the explanation because there are the different layers of the egg and thosecolor layers will be retained to all the different pie chart diagrams that you see. Let's startwith the first step of fertilization. In this particular step, the sperm just needs to get closeenough to the egg. It has a special matrix around the egg that it needs to get through and itdoes this with the special protein called PH-20. This PH-20 allows for the sperm to get closeenough to even touch the egg. At this point, the sperm is going to have to try to breakthrough these couple layers of the egg. The first one is the zona pellucida. This zone is veryimportant. This is the first protective layer that the sperm needs to get through. There are afew different receptors and glycoproteins that need to have an interaction take place. So thesperm needs to bind to ZP3 portions on the zona. At this point, it has specific receptors thatare being expressed after that capsule change that happened on the trip in. If it has thesealigned ZP3 receptors to these glycoproteins on the membrane, you'll have a binding take place.
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Then after the binding takes place, enzymes are released from the cap. These enzymes allowfor some digestion of the membrane of the egg, then the sperm can move in a little bit closerand then has a second binding protein and these are ZP2 binding proteins. As this occurs, itallows the egg head to move just inside the zona. In this next step fertilization, you need toget the sperm just a little bit further through into the egg. This case, there's a little bit ofswimming that happens to get it to that next point or junction. Now the sperm can fuse with theegg membrane and this case is when the DNA is released into the ovum. There is a smallsignaling cascade that happens and this involves an enzyme called phospholipase C.
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Phospholipase C breaks down the enzyme or the substance called PIP2 into IP3 and DAG. Whatthis does is cause a calcium signaling cascade to occur. This signaling cascade is importantbecause what happens is that this cascade elicits responses that modify the ZP2 and ZP3glycoproteins so that no other sperm can bind to the egg. Then what happens to the sperm? Ithas done its job. The flagellum, the mitochondria, the neck region, those all will disintegrate.
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So all you have is the delivered DNA and this is the reason why you don't have DNA frommitochondria in an offspring because this only comes from the mother because the father'smitochondria are not delivered, only the DNA that will eventually be in the nucleus.

About the Lecture

The lecture Fertilization by Thad Wilson, PhD is from the course Reproductive Physiology.

Included Quiz Questions

Which of the following is the most common fertilisation location?

Ampulla

Uterus

Vagina

Fallopian tubes

Ovaries

Which of the following proteins allow sperm to gain proximity to the egg?

PH20

ZO3

PH40

Phospholipase C

PIP2

Which of the following ions modify ZP2 to resist entry of more sperms after the first has succeeded?

Calcium

Magnesium

Sodium

Phosphate

Iodine

Author of lecture Fertilization

Thad Wilson, PhD

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